Epigenetics is the study of heritable changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence. Molecular mechanisms that play a role in epigenetic regulation include DNA methylation and chromatin/histone modifications. Histone methylation, in particular, is critical in many epigenetic phenomena.
Chromatin, the organized assemblage of nuclear DNA and histone proteins, is the basis for a multitude of vital nuclear processes including regulation of transcription, replication, DNA-damage repair and progression through the cell cycle. A number of factors, such as chromatin-modifying enzymes, have been identified that play an important role in maintaining the dynamic equilibrium of chromatin (Margueron, et al. (2005) Curr. Opin. Genet. Dev. 15:163-176).
Histones are the chief protein components of chromatin. They act as spools around which DNA winds, and they play a role in gene regulation. There are a total of six classes of histones (H1, H2A, H2B, H3, H4, and H5) organized into two super classes: core histones (H2A, H2B, H3, and H4) and linker histones (H1 and H5). The basic unit of chromatin is the nucleosome, which consists of about 147 base pairs of DNA wrapped around the histone octamer, consisting of two copies each of the core histones H2A, H2B, H3, and H4 (Luger, et al. (1997) Nature 389:251-260).
Histones, particularly residues of the amino termini of histones H3 and H4 and the amino and carboxyl termini of histones H2A, H2B and H1, are susceptible to a variety of post-translational modifications including acetylation, methylation, phosphorylation, ribosylation, sumoylation, ubiquitination, citrullination, deimination, and biotinylation. The core of histones H2A and H3 can also be modified. Histone modifications are integral to diverse biological processes such as gene regulation, DNA repair, and chromosome condensation.
One class of histone methylases is characterized by the presence of a SET domain, comprising about 130 amino acids. EZH2 is an example of a human SET-domain containing methylase. EZH2 associates with EED (Embryonic Ectoderm Development) and SUZ12 (suppressor of zeste 12 homolog) to form a complex known as PRC2 (Polycomb Group Repressive Complex 2) having the ability to tri-methylate histone H3 at lysine 27 (Cao and Zhang, Mol. Cell 15:57-67, 2004). PRC2 complexes can also include RBAP46 and RBAP48 subunits. Another example is the related methylase EZH1.
The oncogenic activities of EZH2 have been shown by a number of studies. In cell line experiments, over-expression of EZH2 induces cell invasion, growth in soft agar, and motility while knockdown of EZH2 inhibits cell proliferation and cell invasion (Kleer et al., 2003, Proc. Nat. Acad. Sci. USA 100:11606-11611; Varambally et al., (2002), “The polycomb group protein EZH2 is involved in progression of prostate cancer,” Nature 419, 624-629). It has been shown that EZH2 represses the expression of several tumor suppressors, including E-cadherin, DAB2IP and RUNX3 among others. In xenograft models, EZH2 knockdown inhibits tumor growth and metastasis. Recently, it has been shown that down modulation of EZH2 in murine models blocks prostate cancer metastasis (Min et al., “An oncogene-tumor suppressor cascade drives metastatic prostate cancer by coordinately activating Ras and nuclear factor-kappaB,” Nat Med. 2010 March; 16(3):286-94). EZH2 overexpression is associated with aggressiveness of certain cancers such as breast cancer (Kleer et al., Proc. Nat. Acad. Sci. USA 100:11606-11611, 2003). Recent studies also suggest that prostate cancer specific oncogenic fusion gene TMPRSS2-ERG induces repressive epigenetic programs via direct activation of EZH2 (Yu et al., “An Integrated Network of Androgen Receptor, Polycomb, and TMPRSS2-ERG Gene Fusions in Prostate Cancer Progression,” Cancer Cell. 2010 May 18; 17(5):443-454).